The present invention relates in general to electrostatography and, in particular, to a process for reducing image defects an electrostatographic imaging apparatus that contains particulate contaminants.
Electrostatographic imaging apparatus in general and electrophotographic imaging apparatus and techniques in particular have been extensively described in patents and other literature. In general, electrostatographic apparatus comprises a primary imaging member such as a photoconductive element, on which an electrostatic latent image can be formed. The latent image is then developed into a visible image using an appropriate developer contained in a suitable development station. The developed image is transferred from the primary imaging member to a receiver, where it is permanently fixed using a suitable process such as fusing. Alternatively, the image can be first transferred to a transfer intermediate member and thence to the receiver. Such an intermediate transfer member is described in Rimai et al., U.S. Pat. No. 5,807,651, ELECTROSTATOGRAPHIC APPARATUS AND METHOD FOR IMPROVED TRANSFER OF SMALL PARTICLES, the disclosure of which is incorporated herein by reference. In order to produce color images, separate latent images comprising the appropriate color information are produced on the primary imaging member, converted into visible images using developers contained in multiple development stations, and ultimately transferred to a receiver using known methods.
For the specific case of an electrophotographic apparatus and process, the primary imaging member comprises a photoconductive element, which is initially electrically charged using known technology such as a corona or roller charger. An electrostatic latent image is then formed by image-wise exposing the photoconductive element to suitable electromagnetic radiation, using, for example, optical exposure or a laser scanner or LED array. The electrostatic latent image is developed into a visible image by bringing the photoconductive element of the primary imaging member into close proximity to a development station containing a suitable developer comprising toner particles of appropriate color and electric charge.
The development process can be either a discharged area development (DAD) process, in which the toner is deposited on the discharged areas of the photoconductive element, or a charged area development (CAD) process, wherein the toner is deposited on the charged areas of the photoconductive element. In the DAD process, the toner charge generally is of the same polarity as the initial charge on the photoconductive element. The development station is also biased with a potential of the same polarity at a level that is relatively high but lower than the initial charge on the photoconductive element. In the CAD process, the polarity of the toner charge is generally opposite that of the initial charge on the photoconductive element. The development station is biased at a level lower than that of the initial charge on the photoconductive element but generally higher than that residing in the discharged areas of the element.
The primary imaging member comprising the photoconductive element generally also includes a substrate that can be in the form of, for example, a continuous web or a drum. The substrate must itself either be electrically conductive or be coated with a suitable electrically conducting layer such as nickel. The electrically conductive substrate or overcoat layer is then overcoated with a layer that will hold the charge during most of the latent image forming and development process but can be imagewise discharged at the appropriate instances. The substrate overcoat generally comprises a material with photoconductive properties. The primary imaging member can further include suitable additional layers such as, for example, charge transport layers, protective layers such as sol-gels, additional photoconductive layers sensitive to different frequencies in the electrophotographic spectrum, etc.
Development of a latent image formed by imagewise exposure of the photoconductive element is accomplished by passing the element over a suitable development station containing a dry powder developer. It is important that the charge-holding photoconductive layer overlying the electrically conducting layer or conductive substrate be continuous and free of any defects such as xe2x80x9cpin holes.xe2x80x9d While primary imaging members can be manufactured initially free of defects, pin hole type defects of the charge holding layer are known to occur during use. These are generally caused by punctures of the photoconductive layer by contaminant particles. These punctures are frequently found to occur at the transfer station, especially when electrostatic transfer is used and, more particularly, when the electrostatic transfer apparatus comprises a pressure member such as a roller that presses the receiver or intermediate member into contact with the primary imaging member. Although the sources of such particulate contamination are broad, it is frequently found to occur with magnetic carrier particles contained in so-called xe2x80x9ctwo componentxe2x80x9d developers. Other sources of particulate contaminants include carbon or fiberglass reinforcing fibers contained in molded articles within the apparatus, paper filler, etc. Other components contained in the developers, including silica, titania, strontium titanate, barium titanate, etc. can also cause punctures.
The presence of pin holes will result in small discharged areas that do not correspond to the latent image and will produce noticeable defects in the developed image. In the case of the DAD process, an area of the photoconductive element having reduced charge acceptance attracts toner during the development process, resulting in a phenomenon known as xe2x80x9cblack spots.xe2x80x9d Black spots refer specifically to deposits of toner on the latent image in areas or spots that were not discharged solely by imagewise exposure of the photoconductive element. In an image comprising black text on a white background, black spots are manifested as random black dots in the background areas. The term xe2x80x9cblack spotxe2x80x9d is used generically but can refer to spots of any other color toner used in the development of a particular latent image. For example, if the defect occurs in a latent image separation of a full color image comprising the cyan information, the xe2x80x9cblack spotxe2x80x9d will actually be cyan in color.
A similar problem occurs in CAD electrophotographic processes. In this case, an area of the photoconductive element with reduced charge acceptance will fail to attract toner during the development process, even if that area is within the image area that should be toned. This gives rise to a phenomenon known as xe2x80x9cwhite spotsxe2x80x9d. White spots refer specifically to the reduction or absence of toner deposits on the latent image in areas or spots that were not discharged solely by imagewise exposure of the photoconductive element. In the case of an image comprising a black or colored image, white spots are manifested as random white dots in the black or colored areas.
Clearly, the presence in an image of black spots in a DAD system or white spots in a CAD system reduces quality of the image, leading to lowered productivity and increased calls for service. Thus there is a need for a method of preventing black or white spots in electrostatographic image producing apparatus in general and electrophotographic apparatus in particular. The present invention describes a method for reducing image defects resulting from puncture of a photoconductive element at the transfer station of an electrostatographic apparatus prone to particulate contamination. Images with significantly fewer black spots or white spots are thereby obtained.
The present invention is directed to a process for reducing image defects in an electrostatographic image resulting from particulate contamination. The process comprises: uniformly charging a primary imaging member that comprises a photoconductive element and an outermost layer of silicon carbide and is included in an electrostatographic imaging apparatus subject to particulate contamination. The apparatus further includes a charging station, an exposing station, at least one developing station, and a transfer station that comprises an electrically biased roller transfer assembly. The primary imaging member is exposed imagewise at the exposing station to form a latent image on the imaging member, which is developed with toner at the developing station to form a developed image on the imaging member.
The primary imaging member bearing the developed image is passed through the charge erasing station to remove residual surface charge, then contacted with a receiver by the electrically biased roller transfer assembly, causing the developed image to transfer to the receiver. The outermost layer of silicon carbide protects the photoconductive element against damage by contaminant particles present in the apparatus and thereby mitigates generation of image defects in the roller transfer assembly of the developer station.